This invention relates to a front bicycle derailleur fork for controlling the selective engagement of a transmission chain of a bicycle on a plurality of chainrings of different diameters associated to the bottom bracket of the bicycle, said fork comprising an inner plate and an outer plate, which is essentially parallel and distanced with respect to the inner plate, and joined to it by an upper bridge portion, said outer plate being located on the side of the larger diameter chainring and said inner plate being located on the side of the smallest diameter chainring.
FIG. 1 of the annexed drawings shows a perspective view of a front bicycle derailleur of the traditional type, comprising a fixed body 1 with a fork 2 for anchoring to a tube 3 of the bicycle frame near the bottom bracket. The derailleur 1 also comprises a fork 4 for controlling the selective engagement of the bicycle chain (not illustrated) on the chainring (not illustrated) associated to the bottom bracket. The fork 4 comprises an inner plate 5, closer to the bicycle frame, and an outer plate 6, facing outwards, which is parallel and distanced with respect to the plate 5. The two plates 5, 6 are connected by an upper bridge 7 and a rear bridge 8. The upper bridge 7 is provided with a pair of parallel tabs 9 for the articulated connection to a rocker arm 10, which is articulated to the fixed body 1 of the derailleur in point 11 and presents an opposite end 12 for anchoring the end of the derailleur actuating flexible wire (not illustrated). The inner plate 5 also presents a pair of parallel tabs 13 for the articulated connection to a second articulated arm 14, whose opposite end is connected in an articulated fashion to the fixed body 1 in point 15. The two arms 10, 14, consequently, define an articulated parallelogram connection, which allows the fork 4 to move with respect to the fixed body 1 in a direction which is essentially transversal with respect to the longitudinal direction of the bicycle, indicated by the arrow A. A spring 16, which tends to recall the fork 4 forwards an end of travel position (leftwards with reference to the drawing), is provided on the spindle of the inner plate 4 and the arm 14. From this position, the fork can be moved rightwards (always with reference to FIG. 1) following the intervention of a derailleur control (not illustrated) whereby pulling the arm 10 by means of the flexible wire connected to the end 12.
In this description and in the claims which follow, the expressions xe2x80x9cinner platexe2x80x9d and xe2x80x9couter platexe2x80x9d refer to the usual assembly condition of the front bicycle derailleur. According to the traditional assembly arrangement, the set of chainrings associated to the bottom bracket of the bicycle is arranged to the right of the bicycle frame, with the largest diameter chainring arranged on the outside and the smallest diameter chainring arranged on the inside. Similarly, the fork 4 presents an inner plate 5 facing the bicycle frame and an outer frame 6 facing outwards with respect to the bicycle.
It is essential to note that during the front derailleur operation, one of the two plates 5, 6 of the derailleur pushes the chain laterally to shift it from the chainring is it meshed with to the next chainring. More precisely, the lateral pushing action of the chain is exerted by the inner plate 5, when the chain is shifted from a smaller diameter chainring to a larger diameter chainring, and by the outer plate 6, when the chain is shifted from a larger diameter chainring to a smaller diameter chainring.
The objective of this invention is to make a front bicycle derailleur fork which is capable of working efficiently and reliably in all conditions of operation of the derailleur and which is simple and lightweight at the same time.
To attain this objective, this invention relates to a fork of the type illustrated at the beginning of this description, characterised in that said outer plate and said inner plate are two separate elements made of different materials. Preferably, the material of the inner plate is stiffer and harder and the material of the outer plate is lighter.
The fork according to this invention differentiates from those made according to the traditional technique, which are typically made of a single pressed and bent metal part.
Studies and experiments conducted by the Applicant have ascertained that the fork structure can be optimised, both in terms of simplicity and lightness, considering that, for the reasons illustrated above, the outer plate and the inner plate of the fork are called to perform different tasks. In practice, while a relatively stiffer and harder material is needed for the inner plate, since this plate is called to perform the more difficult task of shifting the chain xe2x80x9cupxe2x80x9d from a smaller diameter chainring to a larger diameter chainring, the material chosen for the outer plate can be softer and lighter.
According to a preferred form of embodiment, the inner plate of the fork according to this invention is made of light alloy, while the outer plate is made of fibre-reinforced synthetic resin, for example carbon fibre-reinforced synthetic resin.
In a first form of embodiment, the element forming the outer plate incorporates a rear bridge, connecting the rear ends of the inner plate and of the outer plate. In an alternative form of embodiment, on the other hand, a third element, which is separate from the two elements forming the inner plate and the outer plate, forms said rear connection bridge between the two plates.
The two elements forming the inner plate and the outer plate preferably incorporate respective upper plates, which overlap and are joined to form said upper connection bridge, the elements defining the outer plate and the inner plate being connected by said overlapping plates forming the upper bridge and by the rear ends, either directly, as in the case of the first form of embodiment described above, or indirectly, by means of the third element forming the rear connection bridge, in the case of the second form of embodiment described above.
In an additional form of embodiment of this invention, the inner surface of the outer plate of the fork is provided with an additional plate, projecting from the space between the inner plate and the outer plate, to locally decrease the width of this space, according to the principles specified in Italian Patent Application TO99A000792 by the Applicant.
According to another possible form of embodiment, the outer face of the outer plate can be provided with an additional anti-wear plate, made of metallic material.